Conventionally, a transparent touch panel is used as an input device by disposing it on a display such as a liquid crystal display.
Such a known transparent touch panel is shown in FIG. 4, and adopts resistance films. In this transparent touch panel, a movable electrode film 2 on which a transparent movable electrode 4 has been formed is spaced by spacers 5 at a small interval from a fixed electrode-supporting member 8 on which a transparent fixed electrode 7 has been formed, and the movable electrode film 2 and the fixed electrode-supporting member 8 are bonded to each other such that the movable electrode 4 and the fixed electrode 7 are opposed to each other. In the transparent touch panel adopting this system, electrical continuity occurs across the movable electrode 4 and the fixed electrode 7 by pressing down the movable electrode film 2 with a finger or a pen. The upper surface of the movable electrode film 2 is damaged by repeating such an input operation. Thus, in order to protect the surface of the transparent touch panel, a hard coating layer 1 is often formed on the upper surface of the movable electrode film 2.
The conventional transparent touch panel has, however, a problem that interference stripes appear on the movable electrode film 2 or the movable electrode film 2 is likely to be whitened and thus the transparent touch panel has an unfavorable outlook and visibility, because of the following. That is, in forming the hard coating layer 1 on the upper surface of the movable electrode film 2, the movable electrode film 2 is heated or irradiated by ultraviolet rays to crosslink the hard coating layer 1. The resin of the hard coating layer 1 is not all crosslinked but some resins are not crosslinked only in the process of heating the movable electrode film 2 or irradiating the movable electrode film 2 by ultraviolet rays. In the movable electrode film 2 on which the hard coating layer 1 has been formed, a circuit for allowing electrical continuity across the movable electrode film 2 and the movable electrode 4 is printed with a conductive ink or an adhesive agent for bonding the movable electrode film 2 and the fixed electrode-supporting member 8 to each other is printed in the periphery of the movable electrode film 2. In such printing process, a heating treatment is carried out as a drying process. The hard coating layer 1 and the movable electrode film 2 are thermally contracted by this heating treatment, but the behavior of the hard coating layer 1 and that of the movable electrode film 2 are different from each other. That is, crosslinking of uncrosslinked resin of the hard coating layer 1 proceeds due to the heating treatment performed in the printing process. Thus, the thermal contraction coefficient of the hard coating layer 1 exceeds that of the movable electrode film 2. Consequently, the middle portion of the movable electrode film 2 which has integrally adhered to the hard coating layer 1 is warped in a direction where a recess is formed as shown in FIG. 4, thus becoming abnormally close to the fixed electrode-supporting member 8. As a result, interference fringes are likely to appear around the recess of the movable electrode film 2 toward the edges thereof, thus deteriorating the outlook and visibility of the transparent touch panel.
In addition, a polymer called oligomer which is an uncrosslinkable component of the movable electrode film 2 is deposited on the surface of the movable electrode film 2, and thus it is possible that the surface of the movable electrode film 2 will become withened. Specifically, the movable electrode film 2 is significantly whitened when polyethylene terephthalate is used as the movable electrode film 2. Oligomer is likely to be deposited on the surface of the movable electrode film 2 after heating treatment in the process of manufacturing the transparent touch panel is completed or after environment-resistant property testing is conducted on the transparent touch panel, because the deposition of oligomer is accelerated by heating and humidification. The whitened movable electrode film 2 deteriorates the transparency of the transparent touch panel, thus deteriorating the outlook and visibility thereof.
It is conceivable to thicken the hard coating layer 1 to increase the surface strength of the transparent touch panel. But the degree of the warpage of the movable electrode film 2 in the middle thereof increases in the direction to form the recess with the increase in the thickness of the hard coating layer 1. Thus, there is a limitation on increasing the surface strength of the transparent touch panel.
Accordingly, it is an object of the present invention to solve the above-described disadvantages and provide a transparent touch panel superior in outlook and visibility by preventing interference fringes or whitening from being generated and capable of having a high surface strength.